It is known to use ultrasound to image anatomical structures by transmission and reception of ultrasound waves from a transducer. Anatomical structures that are imaged by ultrasound may include interior structures of the body such as cavities, ducts, lumens and vessels. Interior structures of the body may comprise fluid-filled structures. In some circumstances, interior structures of the body may comprise structures that are normally air-filled but have been filled with a fluid (for example, with saline) for imaging purposes.
Three-dimensional (3D) ultrasound images may be obtained by using software to combine ultrasound data that has been taken at different positions or angles to obtain volumetric ultrasound data.
Other medical imaging modalities (for example, CT, MR, PET or SPECT imaging) may also be used to obtain three-dimensional data that is representative of interior structures of the body. The interior structures may comprise, for example, cavities, ducts, lumens, vessels or fluid-filled structures. Additionally, in some modalities the interior structures may comprise air-filled structures. The interior structures may comprise airways or lung cavities.
In addition, imaging modalities may use some type of contrast medium to enhance particular features. For example, micro bubble contrast may be used in ultrasound, iodine-based contrast may be used in CT, and gadolinium contrast may be used in MR.
Endoscopic views or flythrough views may be used in medical imaging applications to image cavities, ducts, vessels or other interior structures. An endoscopic view may be a view that simulates the view obtained in an endoscopy. An endoscopic view may refer to an examination for which previously, or alternatively, a real endoscope is used. In a real endoscopy, a camera is introduced into an interior structure of the body and the view from the camera is displayed on a screen. In contrast, in an endoscopic view of volumetric ultrasound data, a virtual camera is placed within an anatomical structure (for example, within a cavity) and the ultrasound data is rendered to produce an image of the anatomical structure that is viewed from the virtual camera position.
An endoscopic view may comprise a flythrough view. Additionally, the term flythrough view may be used to include images that display interior structures that are not commonly accessible using physical endoscopic devices.
A flythrough view may comprise an animated sequence of images such that the viewpoint appears to move through the interior structure. A flythrough view may make use of perspective projection such that proximal parts of the anatomical structure appear larger than distal parts. A flythrough view may make use of a fish-eye projection or one of a range of related angular projections.
A user may control navigation through the interior structure, for example by moving a virtual camera position through the interior structure. Flythrough may be used to explore lesions and ingrowing masses. Flythrough may be used to plan and follow up interventions such as placing stents or grafts.
Rendering an image from volumetric data may comprise placing one or more virtual light sources with respect to the coordinate system of the volumetric data and simulating light from the virtual light source or sources.
In some rendering methods, a virtual directional light source is used. A virtual directional light source may be a light that originates outside the volume represented by a volumetric data set that is being rendered, and that lights the volume with parallel rays coming from one direction. When lighting interior structures such as cavities, ducts or vessels, a virtual directional light source outside the volume may not be useful in some circumstances, because all or most of the light from the virtual directional light source may be absorbed before reaching the interior structure. Very little light may penetrate into the area of interest.
In some rendering methods, a virtual point light source is placed inside an anatomical structure of interest, for example inside a cavity. In a flythrough view, the position of the virtual point light source may in some circumstances be difficult to control. Even if the virtual point light source is attached to a virtual camera (simulating the position of the light source in endoscopy), it can be difficult to keep the point light source in a useful and sensible place. Occlusion may force the user to move the virtual point light source in three dimensions in order to obtain good lighting of the cavity. In some systems, input devices may have been constructed for two-dimensional operations and it may be difficult to use such input devices for placing a virtual light source in three dimensions. For example, an input device may be constructed so that a virtual light source position can be moved up and down and from side to side, but not in a perpendicular direction (which may be described as in and out of the screen).
Furthermore, the natural intensity fall-off from a point light (which falls off in intensity according to the inverse square law) may result in an excessive dynamic range in rendered images. Rendered images may be very bright in regions close to the point light source and very dark in regions that are further from the point light source. Structures near the point light source may cause shadowing of structures that are further away. A boundary of the anatomical structure of interest (for example, a lumen wall) may constrain the positioning of the point light source.
FIG. 1 illustrates a possible problem of using a point light with respect to intensity fall-off. A virtual point light source 2 illuminates a cavity 4. An image of the cavity is to be rendered from the position of virtual camera 6. Areas of the cavity that are near to the point source are very brightly illuminated, while areas of the cavity that are far from the point source are much less brightly illuminated. There may therefore be an excessively wide dynamic range in the rendered image.